For many applications of polyethylene, toughness, strength, and environmental stress cracking resistance are important properties. These properties are enhanced when the polyethylene is of high molecular weight. However, as the molecular weight of the polymer increases, the processibility of the resin usually decreases. By providing a polymer with a broad or bimodal molecular weight distribution, the properties characteristic of high molecular weight resins are retained and processibility, particularly extrudability, is improved. A bimodal molecular weight distribution can be explained as follows: in a traditional molecular weight distribution plot (by size exclusion chromotography) of concentrations of species of specific molecular weight vs. log molecular weight, a frankly multimodal molecular weight distribution would show at least two maxima, two maxima being the characteristic of bimodal. The maxima need not be equivalent in magnitude or widely separated.
Three major strategies have been proposed for the production of polyethylene resins with a bimodal molecular weight distribution. One is post reactor or melt blending, which suffers from the disadvantages brought on by the requirement of complete homogenization and attendant high cost. A second is through the use of multistage reactors, which raises questions of efficiency and, again, cost. The third, and most desirable strategy, is the direct production of a broad or bimodal polyethylene via a single catalyst or catalyst mixture in a single reactor. Such a process would provide component resin portions of the molecular weight distribution system simultaneously in situ, the resin particles being intimately mixed on the subparticle level.